A method of optimizing the behavior of an electronic force-measuring device is disclosed. A force-measuring device, in particular a balance, is disclosed that is suitable for carrying out the inventive method.
The behavior of an electronic force-measuring device, for example a balance with electromagnetic force compensation (also referred to as magnetic force restoration or MFR balance) or a balance with an elastically deformable element and strain gauges (also referred to as S/G balance), and thus the accuracy of the measurement results delivered by the force-measuring device, is determined by numerous influence factors. Significant among these influence factors are those described in [1], EP 0 945 717 A1, and in [2], “Bauen Sie Ihre Qualitat auf solidem Grund!” (Build your Quality on Solid Ground!), company publication, Mettler Toledo GmbH, January 2001, as well as in [3], “Wagefibel” (Weighing Primer), Mettler Toledo GmbH, April 2001. One can distinguish between internal influence factors, i.e., those that are determined by the properties of the components of the force-measuring device, and external influence factors, i.e., those that are determined by external physical factors as well as by actions of the user. Further of significance is the behavior of the force-measuring device in the presence of the afore-named internal and external influence factors, which behavior is determined by processes that are implemented in the signal-processing unit of the force-measuring device and controlled by at least one processor.
Properties of the balance that are connected to internal influence factors and play a part in determining the accuracy of a measurement include creep, hysteresis, linearity, eccentric load errors, repeatability, temperature stability, settling time, and resolution.
As discussed in reference [3], the external influence factors that determine the accuracy of a balance are determined by physical factors such as vibrations, temperature effects, air drafts, moisture absorption or moisture release of the weighing load, and electrostatic or magnetic interaction. It therefore can be important to choose the location where the balance is set up so that physical disturbance factors are avoided as much as possible. Further of importance for the measurement accuracy are factors related to the operation of the balance, for example setting the balance into a leveled position, the correct way of putting the balance into operation, the arrangement of the draft shield elements, the selection of an appropriate weighing container, and the positioning of the load on the weighing pan.
The internal and external factors described so far therefore determine the condition of the balance on which the measurement accuracy depends.
The manufacturer of the balance aims to continuously improve the properties of the different structural elements and signal-processing modules of the electronic force-measuring devices in order to eliminate, correct and/or compensate for internal and external disturbance factors that affect these elements and modules.
In order to eliminate internal factors, the optimization is directed at the load cells with their guiding constraints, coupling and pivoting elements, or the force/displacement transducers with the appurtenant sensor devices. In a force/displacement transducer, the linearity of the relationship between force and displacement travel can be of importance, wherein one aims to achieve reproducible elastic properties. As described in reference [1], specific requirements can include that the amounts of anelasticity, creep, and (mechanical) hysteresis should be as small as possible.
Measures that are taken to reduce or prevent external factors include for example a draft shield or vibration-damping mechanical elements.
In order to correct or to compensate internal and external factors affecting the measuring signal in spite of the foregoing measures, balances are equipped with a signal-processing unit with modules by means of which the signal delivered by the measurement transducer is filtered, corrected or compensated. Under ideal circumstances, measurement errors caused by creep, hysteresis, non-linearities, temperature gradients, vibrations and shocks, or electrical interference, etc., are thus completely eliminated.
A balance in which the drift phenomena caused by creep are corrected by a compensation for the drift-related components is disclosed in [4], U.S. Pat. No. 4,691,290. In the method that is used in this balance, a representation of the measured load and the status of the creep are determined and combined with each other in order to arrive at a measurement value that corresponds to the applied load, wherein the creep-related error components are compensated.
Furthermore, according to [4] the mathematical representation of the status of the creep is determined as a function of time, of the load being measured, and of the creep status that was determined at an immediately preceding time, in order to take factors into account that affected the balance previously.
The creep status in this balance is calculated based on constants that were determined and stored in the initial adjustment of the balance. According to [4], these constants need to be determined individually for each balance, because there are in most cases variations between different balances.
Important in balances with electromagnetic force compensation, to name an example, is the compensation of temperature drifts that occur after switching the balance on, as described in reference [6], U.S. Pat. No. 5,856,638.
In balances that are disclosed in reference [7], US 2004/0088342 A1, and reference [8], U.S. Pat. No. 6,271,484 B1, signals delivered by the measuring transducer are processed by means of variable digital filters.
With the method described in [7], the characteristics of the filter being used can be individually adapted to the oscillatory properties of the measuring system that is controlled by the filter. Therefore, the damping of the filter can be increased to any desired degree in a selected frequency range.
According to the method described in [8] a test is made as to whether the amplitude of the disturbance signals caused by vibrations lies inside a permissible range. If this is not the case, the filter characteristic is modified until the disturbance signals are again inside the permissible range.
There can be an interactive connection between internal and/or external influence factors as sources of disturbances. If a balance is set up at a location that is not optimal in regard to drafts, and if the draft shield on the balance was not optimally configured by the user, the occurrence of air drafts can affect the measurement results. As an additional factor in this, the effects caused by the air drafts are also dependent on the resistance that the load on the balance pan presents to air currents. The effects of air currents can therefore by reduced by selecting a more favorable location, by configuring the draft shield correctly and/or by selecting a more advantageous container for the weighing load. Furthermore, there is often also a load-dependent shift in the resonance frequency of the mechanical measuring system, which can be taken into account in setting the filter parameters.
The effects caused by the internal and external influences can therefore to a large extent be eliminated by the measures of the foregoing description. However, under very unfavorable conditions of the balance which occur, e.g., as a result of strong mechanical shocks or exposure of the balance to strong air drafts without a draft shield in place, it is often impossible to achieve a compensation for the influence factors or a correction of the resulting measurement error.
The disclosures of reference [1] to [9] are hereby incorporated by references herein in their entireties.
It has been found that in spite of the afore-described measures, it is still possible that unsatisfactory measurement results can occur which are caused for example by either a gradual or abrupt change in the afore-described internal or external influence factors.